Interview: How to get a chip-sized spacecraft to Alpha Centauri and back

E&T: How did the idea come about?

Pete Worden: I met Yuri Milner during one of the Breathrough Prize foundation’s ceremonies about five or six years ago when I was still the director of Nasa Ames. We talked and discovered we had some shared interests – for example whether there is intelligent life elsewhere in the Universe. He was very interested whether we could theoretically go to habitable planets around the nearby stars and see what’s there. We started working together. I looked into the existing concepts that Nasa had been developing and found that doing something like this would require some ridiculous extrapolation of technology and a huge amount of money. So about two years ago, we asked Professor Avi Loeb, the chair of Harvard University’s Astrophysics Department to put together a study team and give us some alternative ideas.

E&T: What did they find?

Pete Worden: They came up with an idea which was to use Earth-based lasers to push a sail in space. Originally, the idea required an immensely powerful laser array that would take up 10 square kilometres and that didn't look feasible. We started looking at technology to get a large laser beam through the atmosphere using atmospheric compensation and eventually we concluded that although very difficult, it was feasible.

It was around the time when Stephen Hawking was in the USA for the inauguration of the Black Hole Institute at Harvard University. We talked to him about it and he was very excited. He did some calculations himself and said he thought it looked good. We asked a number of other Nobel laureates and Breakthrough laureates and other knowledgeable people and the conclusion was that this was a challenge, but we should start researching it.

E&T: What are the hardest problems to solve?

Pete Worden: First of all, we would need to build a laser array one square kilometre is size and the question is whether this is feasible. Secondly, we have to find a material for the sail that would carry this tiny chip to Alpha Centauri, which is 4.37 light years away. We have to put 50 GW of power on the sail, which will be about four metre in size. The material has to have the right reflectivity, minimum absorption. Finally, we need to have an integrated system concept.

So what we plan to do now is to spend about five years, using the $100m that Yuri Milner made available, addressing these key technical issues. If we find that the whole think makes sense, we would raise more money privately for a prototype system and assuming that works then we would look for a public-private partnership and maybe within 20/25 years construct the Starshot system.

E&T: How would the whole thing work?

Pete Worden:  We would build this kilometre laser array probably in the Andes, because Alpha Centauri is in the southern hemisphere. It would have some sort of energy storage system – a battery, flywheels. We would charge the system up all 24 hours and then for two minutes we would fire it into space. Before that, we would put a satellite into highly elliptical orbit. This satellite would weigh maybe a couple of thousand of kilograms. It would carry thousands of these chips, each attached to a sail and it would deploy one at the time into space. We would aim the ground-based laser at the sail and fire for two minutes, it would push the sail at about 60,000 Earth gravities, a very high force, that’s why you can’t put any living things on this. Then in those two minutes it gets to 20 per cent of the speed of light. Over the course of a year, we would push hundreds of thousands of these chips towards Alpha Centauri.

E&T: How long will it take them to get there?

Pete Worden: They will travel for 20 to 25 years in the interstellar space. When they reach the target, they would open up and point the sail, which would now act as a telescope. There is a small camera on the chip, which would take pictures of the planet. We would get high resolution images and we would see how it looks like there and maybe even signs of civilisation. We will measure other things, like the magnetic field and so on, and after we pass through the system, which happens very quickly, in less than a day, we then turn the sail back towards the Earth. The chip is equipped with a small laser - less than a watt. It encodes the laser communication signal, which is then fired back towards the Sun. And four and a half years later we would use the transmitting array now as a receiver, a giant telescope, to receive the photons and then we would get the signal from there with the images.

E&T: Why do you want to go to Alpha Centauri?

Pete Worden: We obviously had to figure out whether there is a place where to send these tiny spacecraft, whether there are any potentially life-baring planets around the nearest stars. So we launched another project, called Breakthrough Watch, which is looking into that.

As serendipity would have it, scientists have actually detected what might be an Earth-sized planet in a habitable zone recently in the solar system of Proxima Centauri. Proxima Centauri is a star in the Alpha Centauri system and actually the nearest star, so we now have some place to go and we will be doing more work on that. It will obviously take a lot of time to get any data. If I am lucky, I will be still alive when the spacecraft is launched and then it would take another 30 years, to get the images back.